The present invention relates generally to floating offshore production vessels. Goldman (U.S. Pat. No. 4,995,762); Hunter (U.S. Pat. No. 5,439,321); Danzacko (U.S. Pat. No. 4,913,238); Meyer-Haake (U.S. Pat. No. 4,217,848); Horton (U.S. Pat. No. 4,702,321), (U.S. Pat. No. 4,740,109) disclose offshore floating vessels of various configurations, all incorporated herein by reference. In these and other conventional vessels, risers running from the well head to the drilling or production equipment are supported by a buoyancy apparatus which either directly supports the risers with a floating vessel, or indirectly supports the risers with individual buoyancy cans, or some other means such as hydraulic cylinders attached between the vessel and the risers.
Offshore environmental conditions are often harsh. Because the buoyancy apparatus is supporting the risers, these risers are directly subjected to the wave action on the buoyancy apparatus. This puts strain on the risers.
Furthermore, wave action attenuates with depth. Therefore, there is less wave action at 500 feet than there is at the surface. Thus, the riser at the sea floor experiences virtually no wave and current action, while the same riser at the surface of the water experiences very harsh wave and current action. Even further, the buoyancy apparatus itself, experiences different wave and current action at the top of the buoyancy apparatus than at the bottom of the buoyancy apparatus.
Even further, many conventional buoyancy apparatuses have short natural periods. For example, conventional tension leg platforms, have a natural period in the three to four second range. Such a short natural period can cause resonance problems such as springing and ringing.
Therefore, there is a long felt need for a buoyancy apparatus that protects the risers from wave action at the surface, is designed to compensate for varying wave action with depth, and has a longer natural period.